1. Field of the Invention
The present invention relates to an external cavity resonator type tunable light source, and more particularly, to an external cavity resonator type tunable light source which can be easily fabricated and which uses a technique enabling wavelength sweeping at a high speed.
2. Description of the Related Art
As is well known, a tunable light source is used for evaluating optical communication lines, optical communication devices and fiber brag grating (FBG) sensors.
As a tunable light source of this type, an external cavity resonator type tunable light source called a Littman type is known.
The external cavity resonator type tunable light source basically has a structure shown in FIG. 10.
A tunable light source 1 shown in FIG. 10 has the following structure. That is, the light emitted from a low reflectance facet in a semiconductor laser 2 to which an anti-reflection (AR) coating have been applied is converted into parallel light beams by a collimator lens 3. The converted parallel light beams are made incident to the side of a diffraction face 4a of a diffraction grating 4 for diffracting light. The diffraction light beams diffracted by the grating 4 relevant to the incident light beams are made incident to a mirror 5. The reflection lights reflected on the mirror surface are made incident again in a reverse optical path to the diffraction grating 4, and the diffracted light beams relevant to the reflected light beams are returned to the semiconductor laser 2. An external cavity is formed between another face of semiconductor laser 2 and the reflection face.
In the tunable light source with this structure, among wavelength components of the light beams diffracted by the diffraction face 4a after being emitted from the semiconductor laser 2, only a specific wavelength perpendicular to the mirror surface 5a and its proximal wavelength components are returned to the semiconductor laser 2.
The semiconductor laser 2 amplifies the returned light having a specific wavelength to produce a standing wave in the external cavity, and emits a light beam having its specific wavelength (referred to as an external cavity resonator wavelength).
The external cavity resonator wavelength is specified by both of an angle formed between the diffraction face 4a and the mirror surface 5a and an optical path length reaching the mirror 5 from the semiconductor laser 2 through the diffraction grating 4. Thus, an angle (or distance) of the mirror surface 5a with respect to the diffraction face 4a is changed, thereby making it possible to change its resonance wavelength.
Then, a plane H1 extending the face 4a of the diffraction grating 4, a plane H2 extending an effective end facet of the resonator (end face considering a refractive index of the semiconductor laser 2 and the collimator-lens 3), and a plane H3 extending the mirror surface 5a cross one another at an identical position O, and the mirror 5 is rotated in a direction indicated by the arrow A around the position O, whereby the external cavity resonator type tunable light source can vary a resonance wavelength continuously.
In the case where the external cavity resonator type tunable light source having such a basic structure is actually provided in a variety of devices, it is necessary to support the mirror 5. In addition, since a support member for making a turning operation around the predetermined position O crosses an optical path going from the semiconductor laser 2 to the diffraction grating 4, it is necessary to configure the support member so as not to block the optical path.
In an example of a structure of the support member, according to a first prior art, as shown in FIG. 11, the support member is turnably supported at the one end side, in a direction indicated by the arrow B, and, at the other end side, a light transmission hole (or a cutout portion) 6a is provided at an intermediate portion of an arm shaped support body 6 for supporting the mirror 5. The emitted light of the semiconductor 2 or the light from the collimator lens 3 is passed through the hole 6a. 
The above-described example of the structure of supporting the mirror 5 by the support member 6 according to the first prior art is disclosed in patent document 1 (U.S. Pat. No. 5,319,668).
In addition, as a support member for supporting and turning a mirror without blocking an optical path, according to a second prior art, there is known a support member for turning a mirror 5 in a state in which the mirror is vertically stood on a movable section 12 which is formed using a flat wafer 7 and which is turnable along the one face thereof, as shown in FIGS. 12 to 16.
The wafer 7 called as SOI (silicon on insulator) wafer which consists of two silicon substrates 8, 9 and an insulation film (SiO2) 10 as shown in FIG. 12, and movable section 12 is made by etching process for the upper substrate 9.
A fan shaped hole 11 is formed in the upper substrate 9, and, a fan shaped movable section 12 formed in the fan shaped hole 11 is formed inside the hole. An insulation film 10 at a bottom face of the hole 11 is removed by etching, and a surface of the lower substrate 8 is exposed.
The movable section 12 has: two plate spring portions 13, 14 extend in a small width from a narrower arc shaped edge part of the hole 11 to a wider edge part thereof, and which can be bent in a plane parallel to the lower substrate 8 and orthogonal to its lengthwise direction; a wider disk plate 15 connecting the tip ends of the plate springs 13, 14 in an arc shape; and an electrode portion 16 which extends from an inner edge of the disk plate 15 toward a narrower arc shape edge part of the hole 11. On both sides of the electrode portion 16, combs 16a, 16b are protruded in an arc shape at predetermined intervals.
The movable section 12, as shown in FIGS. 14 and 15, is supported in a state in which it is slightly floated from a top face of the lower substrate 8. The disk plate 15 and the electrode portion 16 of the movable section 12 can be turned in a direction indicated by the arrow C on a face parallel to a top face of the lower substrate 8 inside of the hole 11 by lateral bending of the plate spring portions 13, 14.
In addition, stationary electrodes 17, 18 are allocated, respectively, between the plate spring portion 13 and the electrode 16 and between the plate spring 14 and the electrode 16.
The stationary electrodes 17, 18 are fixed onto the lower substrate 8 via the insulation layer 10 while these electrodes are insulated from the upper substrate 9. The electrodes have arc shaped combs 17a, 18a formed so as to be mated in a state in which a gap is provided to each of the combs 16a, 16b of the electrode portion 16 of the movable section 12.
Although the mirror 5 rotating mechanism fabricated in the wafer 7 shown in FIGS. 12 to 15 is shown as having the simplest structure, a plurality of electrode portions 16 may be provided, and the stationary electrodes 17, 18 may be provided for each of the electrode portions.
In the case of the thus configured mirror rotating mechanism, for example, as shown in FIG. 16, when a predetermined voltage V is applied between the movable section 12 and the stationary electrode 17, an electric field is generated between the comb 17a of the stationary electrode 17 and the comb 16a of the electrode portion 16 of the movable section 12. Then, an electrostatic force in an attractive direction is generated between the combs, and the electrode portion 16 is attracted to the side of the stationary electrode 17. Then, the whole movable section 12 turns in a counterclockwise direction (indicated by the arrow D) shown in FIG. 16, and stops at a position at which the attractive force and the reaction of the plate spring portions 13, 14 are well equilibrated.
The stop position of the movable section 12 can be arbitrary varied in a predetermined range by varying the applied voltage V.
Therefore, as shown in FIG. 12, the mirror 5 is fixed onto the movable section 12 in a vertically stood state, whereby an angle of the mirror 5 with respect to the diffraction grating 4 can be varied in a predetermined range without interfering an optical path from the semiconductor laser 2 to the diffraction grating 4, and a wavelength of the light beam emitted from the semiconductor laser 2 can be varied with a downsized construction.
The structure of the movable section 12 for supporting the mirror 5 in a state in which the mirror is stood, according to the second prior art, is disclosed in, for example, patent document 2 (Brochure of International Patent Application Publication No. 01/43241).
However, the support member 6 according to the first prior art disclosed in the above patent document 1 requires a light transmission hole 6a at its intermediate portion, and is structurally complicated. In addition, the structure is prone to be lower in strength, and is easily deformed as compared with a hole free structure, thus making it difficult to vary a wavelength at a high speed.
Further, the movable section 12 fabricated in the wafer 7 according to the second prior art disclosed in the above patent document 2 has a four-point link structure in which both ends of the two plate spring portions 13, 14 are defined as fulcrums. Thus, strictly, the turning center is not constant, and the turning center is displaced more remarkably as the variable angle is increased.
In order to eliminate this displacement of the turning center, it is necessary to make complicated position control relevant to the movable section 12, making it difficult to vary a wavelength at a high speed. In addition, in order to make such complicated control, it is necessary to further make position control of the movable section 12 by using a complicated electrode structure or to make posture control at the diffraction grating side, thus further making the structure complicated.
Moreover, the mirror 5 must be fixed precisely vertically on the movable section 12 of the support member 7, and the fixing work becomes very complicated.